Integrated circuits (ICs) are ubiquitous in today's society. ICs have microscopic dimensions, which may make ICs difficult to test and characterize. Recent techniques for measuring IC parameters include Picosecond Imaging Circuit Analysis (PICA). In general, PICA techniques provide a measure of the relative timing and source position of infrared light or photon emissions that occur at various locations across an IC at various times. The PICA technique relies on current-carrying transistors emitting infrared light as the transistors switch states. Because current flow is indicative of a switching transition, and because infrared light emission is indicative of current flow, photon detection may be used to measure the timing of switching events in digital circuits.
However, accurately extracting spatial-related and timing-related parameters from PICA measurements is not a straightforward procedure. Photon emission is a statistical process, and the probability of detected photon emission for current very large scale integration (VLSI) technology is on the order of 10−9 infrared photons per switching event per micron of transistor channel width. To acquire a statistically useful measurement, many switching events must be generated, and the timing of the separately detected photons must be correlated with a repeatable trigger. The relative timing of the photons can be plotted as a histogram, and the peak of the histogram may be used to determine the timing of the transistor's switching transition relative to the trigger. Unfortunately, the histograms are “noisy” as a result of the granularity of the events (i.e., either a photon is detected, or not detected) and the rarity of events. The noise makes the measurements difficult to interpret. The histograms gradually approach a smooth curve if the measurements are taken over a sufficiently long period of time, but this is feasible for small width devices or devices that switch infrequently relative to the clock frequency. Furthermore, as device sizes continue to decrease, histograms representing spatial photon emission will tend to become less precise.